Antiandrogens are often combined with radiation therapy (RT) or surgery as neoadjuvant, adjuvant or concurrent therapy in an attempt to decrease disease relapse and improve survival. However, the optimal combination and timing of antiandrogen therapy remains controversial. The Radiation Therapy Oncology Group (RTOG) recently activated a Phase III clinical trial, RTOG 99-10, to study the optimal duration of neoadjuvant total androgen suppression (TAS) and RT in stage II-III prostate cancer. However, it is possible that diminished QGL, particularly related to sexual function, will accompany the improved clinical outcomes associated with TAS. Specifically, TAS and RT have a significant impact on erectile function. If interventions are available to treat erectile dysfunction (ED) after prostate cancer therapy, QOL impairments may be minimized. Sildenafil (Viagra TM, Pfizer) is a FDA approved drug for the treatment of ED. Small, nonrandomized; single institution studies have shown significant improvements in ED in men treated with sildenafil after RT. However, the effectiveness of sildenafil for ED as a consequence of combined treatment with RT with TAS has not been adequately described. Conventional wisdom held that after RT plus neoadjuvant or concurrent TAS, erectile function would be equivalent to that after RT alone (once the TAS are discontinued). However, we observed a higher ED rate with the combination of RT and TAS (even after the antiandrogens were discontinued) compared with RT alone in our preliminary work. Antiandrogens affect erectile function through a different mechanism than the etiologies previously studied. If sildenafil is efficacious in this setting, it may assist in both patient decision-making regarding choice of therapy and in clinical management of post-therapy ED. Since most men with prostate cancer have partners, a critical part of the sexual experience, currently lacking in the literature, is an assessment of relationship factors that may interact with ED therapy to predict or modify response to treatment. This knowledge would allow for more effective treatment approaches based on a clinical strategy that provides instruction both on the technical use of the medication as well as on the importance of creating an appropriate psychosexual environment. This study targets men (N=332) recruited for participation from RTOG 99-10, treated with either 8 or 28 weeks of neoadjuvant TAS and RT with concurrent (8 week duration) TAS. The primary aim of this study is to determine, in a randomized, double-blind crossover study, if there is a significant difference in erectile function between men treated with sildenafil versus placebo after RT plus neoadjuvant TAS for prostate cancer. The secondary aim is to determine if there is a significant difference in overall sexual function and satisfaction between men treated with sildenafil versus placebo, and the third and fourth aims are to assess differences in partner sexual satisfaction and dyad marital adjustment between the sildenafil versus placebo arms of this study. Lastly, this study will assess factors associated with response to ED therapy such as age, pre-treatment sexual functions, and tobacco use among others.